The producing of materials in a magnetic field—in particular a so-called “intense” magnetic field i.e. whose intensity is of the order of several Tesla even several tens of Tesla—is the subject of numerous scientific investigations. For example, a branch of activities called “magneto-science” has emerged which sets out to combine the application of a magnetic field with a method for producing a material. The magnetic field is then considered to be an additional parameter which may influence either the morphology of the material being produced or the kinetics of the production methods used, in the same manner as parameters such as temperature, pressure or chemical composition. In this respect, the magnetic field can be used to modify the properties of use of a material. While numerous magnetic field effects are still the subject of fundamental research, others are currently already involved in industrial processes for the synthesis of materials.
The invention developed herein targets both a research and development environment and the industrial environment. In particular, it is desired to be able to use a magnetic field to impact the microstructure and hence the characteristics of a material, as an alternative to means already widely optimized for many years in metallurgy such as variations in chemical composition, the combined use of thermomechanical treatments (hot, cold deformation) and intermediate heat or chemical treatments. Under the effect of a sufficiently intense magnetic field, i.e. of intensity typically higher than 1 Tesla, magnetic energy is no longer negligible compared with the chemical energy involved in the different types of transformations encountered in a material throughout its production. This is the reason why transformation kinetics and microstructures can be modified through the application of a magnetic field.
In metallurgy, the properties of use of an alloy strongly depend upon the history of its production. Therefore, to examine this history and in particular to observe structures that are stable at high temperature, it is necessary to halt the changes in the microstructure at different stages of its formation. This is achieved via quenching to set the microstructure of the alloy at ambient temperature.
This method allows ex-situ quantitative analysis of microstructures. This analysis, coupled with in-situ measurements of transformation temperature is used to determine phase diagrams or other types of predictive diagrams such as TTT diagrams (Time-Temperature-Transformation) or CCT (Continuous Cooling Transformation) diagrams. TTT diagrams are used to examine the kinetics of phase or state transitions. This type of diagram is obtained with step quenching experiments followed by a given temperature hold, for ex-situ microstructural characterization. The transformation rate can then be measured. CCT diagrams are used to predict the microstructure of a solid subjected to thermomechanical treatments. They show the different states through which a given alloy grade may pass on cooling. They correspond to cooling conditions close to those of industrial conditions. Also, the microstructures of most interest for industrial applications very often involve non-equilibrium structures.
It is therefore necessary not only for research purposes but also for industrial applications to be able to examine and make use of the effect of a magnetic field on the formation of any type of microstructure, and in particular of these non-equilibrium structures. However, it is not possible at the current time to perform quenching under the simultaneous effect of a static magnetic field.
Conventionally, quenching in a liquid medium requires the displacement of the treated test-piece towards a medium dedicated to quenching thereof. Yet in a magnetic field, any movement of a conductive or magnetic material generates strong stresses on the device generating the magnetic field. Firstly an electric charge q, moving in a magnetic field B, at velocity v, is subjected to Lorentz forces denoted d{right arrow over (F)} which oppose the movement which set them up:d{right arrow over (F)}=q·{right arrow over (v)}{right arrow over (B)}Secondly, a conductor of length dl, in which an electric current passes of intensity I, in a magnetic field B, is subjected to Laplace forces d{right arrow over (F)}, as per the equation:d{right arrow over (F)}=I·d{right arrow over (l)}{right arrow over (B)}Therefore, two magnetic systems (i.e. the ferromagnetic material and the generator winding) are coupled via mutual induction.
The movement of a ferromagnetic material may therefore perturb, even damage, the magnet supplying the field which is then subjected to possibly major mechanical forces. To conduct quenching, the processes developed up until now consist of extracting the material from the furnace, in which it is subjected to the magnetic field, and immersing it in a quench bath which is located outside the magnetic field. With this process, complex to perform on account of the restricted available space, the magnetic field applied to the material is not constant throughout the entire treatment. The transfer of the material (from the area where the field is applied to the zero-field area) firstly forms a variation in the field applied to the material during its treatment, and secondly the material is no longer subjected to the field when it is being cooled. In addition, this method may be detrimental to the magnet supplying the field.
In U.S. Pat. No. 5,535,990 published on 16 Jul. 1996, an apparatus was proposed allowing the heat treatment of a test-piece whilst applying a magnetic field thereto by means of coils wound around the test-piece to be treated. Said apparatus does not however allow the application of an intense magnetic field, i.e. higher than 1 Tesla, to the test piece and it cannot in any way be adapted for this purpose. In addition, the arrangement proposed in this patent has a certain number of disadvantages, in particular in terms of wear of the apparatus, since the winding used undergoes the same heat treatments as the test-piece.
One alternative for obtaining rapid cooling of the material in the presence of an intense magnetic field consists of sending a gas flow in the direction thereof (e.g. argon or helium) under pressure and at ambient temperature. However, this solution does not allow sufficiently rapid cooling of the material that could be likened to a quench. Therefore, the cooling rates thus obtained do not exceed 50° C./s between 1000° C. and 500° C. and are much lower at lower temperatures when the cooling property of the gas becomes negligible. With quenching in a liquid bath, the cooling rates are globally constant over all temperature ranges and may exceed 150° C./s with good bath sizing.
A first objective of the invention is therefore to define a method allowing the performing of the entire heat treatment (i.e. heating and quenching in a liquid bath) or at least the quench step under the influence of a static magnetic field. Also, in addition to the quench just mentioned, it is envisaged to apply other treatments to the material at high temperature under the effect of a magnetic field. For this purpose, another device is substituted for the quench bath. Amongst the envisaged treatments, mention may be made of surface treatments in salt bath, thermo-mechanical treatments (rolling, forging), etc. A second objective of the invention is therefore to define a method and associated device which more generally allow the performing of at least a step to apply a thermal shock, thermomechanical treatment and/or chemical treatment to a material under the effect of a static magnetic field, truly adaptable on an industrial scale for substantially continuous treatment processes for example.